Increased hepatic metabolism w/ hydantoins (eg, phenytoin), barbiturates (eg, phenobarb), primidone, carbamazepine, rifampicin, oxcarbazepine, rifabutin, topiramate, felbamate, ritonavir, nelfinavir, griseofulvin & products containing St. John's wort. Plasma & tissue conc may be affected (eg, cyclosporine).